There are four methods of stunning hogs today. Firearm, captive bolt, controlled atmosphere stunning (CAS) and electrical. There are advantages and disadvantages to each. Let’s evaluate the four methods.
Firearm and Captive Bolt
The advantage for the firearm and captive bolt methods is the low initial investment of equipment. The small processor can purchase a firearm for a relatively minimum investment. The cost of purchasing bullets is also very low.
The disadvantage is the complying with USDA Humane Slaughter Act; that is, 100% insensibility of the animal with a single shot. There are two factors involved. The first is obviously the human factor. Nobody is perfect. The second is in the physical makeup of the hogs. Variation in breeds and age affect the specific location of their small brain and the slope of their forehead. The operator may target the same location on every animal but the results will not always equate to insensibility in every animal.
Another disadvantage is the amount of leg movement and kicking after the animal is shot. It is not an easy task to shackle an animal with excessive leg movement.
Controlled Atmosphere Stunning (CAS)
The main advantage is the quality of the meat. CAS stunning has the proven results of minimal blood spots and broken bones. Another perceived advantage is that it is humane. The hogs go into chamber. Carbon dioxide gas is injected. Seconds later the animal is ejected from the chamber unconscious. However, most if not all chambers are windowless. What the hogs experience during the time they are inhaling the carbon dioxide can only be evaluated through some on line videos that seem to indicate that the term humane slaughter may not be an accurate description in this method.
The disadvantage, especially for the small and very small processor, is the high initial cost of investment. There can only be a reasonable payback period in large, high volume operations.
This is a popular choice among small and very small processors. Despite limited financial resources, electrical stunners are affordable for small operations. The first advantage is that when electric stunning is done correctly, the animal will feel nothing. The animal will feel nothing provided that the stunner can produce a seizure when applied for 1 to 3 seconds. To prevent pain to the animal, the wand or tongs must be pressed firmly against the animal and directly behind the ears before the button is pushed.
One manufacturer of stunners ensures this by not having a button. The contactors are not energized until both are firmly placed on the animal.
Some processors still experience three problems with electrical stunning. One is excessive leg movement after stunning, short time available for shackling and hoisting prior to bleeding, and thirdly, broken bones and blood spots. Fortunately, these problems can be solved with the correct choice of electrical stunners and the proper stunning technique. The correct choice of stunner is a constant amperage stunner and the proper stunning technique is a two phase stunning method; first, a head to head stun followed by a head to heart stun.
Constant Amperage Stunners
The best electric stunners and the correct choice are the units that have an amperage-regulated circuit. These are commonly referred to as constant amperage stunners. These stunners are superior because amperage is what induces sensibility in hogs and sheep in electrical stunning not voltage.
Scientific research has shown that an electrical stunner must have sufficient amperage to induce a grand mal seizure to ensure that the will be made instantly insensible. For market weight hogs a minimum of 1.25 amps is required. To deliver sufficient amperage, 250 volts is recommended to ensure insensibility. Hogs will vary in fat thickness so an electric stunner that is voltage regulated will apply varying amounts of amps at the same voltage setting as the fat thickness varies. Grandin has noted that the amperage required to induce sensibility for sheep is 1.0 amps and for hogs 1.25 amps. The American Veterinary Medical Association indicates heavier hogs may need more than 1.25 amps.
Older style circuits are voltage regulated. These circuits are inferior because they allow large amperage surges which can fracture bones and cause blood splash. In the U.S. and other countries, these older voltage regulated units are still being used. In very small U.S. plants, these units are popular because they are less expensive. With this type of unit, the desired voltage is set and the amperage varies. Factors (or types of resistance) that result in amperage surges in voltage regulated stunners are:
- Fat thickness
- Access to water prior to stunning
- Wetness of the skin
- Mineral Content of water on the skin
- Wool or Hair Coat Length
- Skin Thickness
The use of these voltage regulated units results in inconsistencies in insensibility along with broken bones and blood spots.
Two Phase Stunning
Two phase stunning begins with the initial application of the contactors on the end of the tongs directly behind the ears until the animal is unconscious (approximately 3-4 seconds).
Head to Head Stunning
Head only stunning affects only the brain. Head only stunning is often used by very small processors and requires more training and labor than the use of two phase stunning. Using solely head to head stunning the animal is only unconscious for 15 to 20 seconds.
The second phase is the head to heart stunning. One contactor is placed on the head while the second is placed on the heart region. Applying the head to head and then the head to heart results in minimal
Head to Heart kicking of the legs and less movement of the
Stunning insensible hogs so the hogs are easier to
handle during shackling and hoisting. This is especially a problem with slow moving hoists.
Small processors should electrically stun in groups. Warriss, et al. (1994) found that pigs were more stressed in operations with single file races compared to plants where pigs were stunned in small groups on the floor. The intensity of squealing was highly correlated with physiological stress measurements and PSE.
Electrical stunning of pigs on the floor is most practical for operations that slaughter under 240 pigs per hour.
In conclusion, if you are a small processor that slaughters hogs and sheep, the most advantageous method of stunning is electrical. Utilize a constant amperage stunner where you can apply the correct amperage for species and size. Use the two phase stunning technique; head to head and then head to heart.
Grandin, 2015 Electric Stunning of Pigs and Sheep
Warriss, et.al. 1994 Relationships between subjective and objective assessments of stress at slaughter and meat quality in pigs. Meat Sci. 1994;38(2):329-40. doi: 10.1016/0309-1740(94)90121-X
AVMA Guidelines for the Humane Slaughter of Animals: 2016 Edition
Maddock, 2011, Meatingplace Stunning Technology Improvements for Pork